Recently, battery-operated portable electronic devices having USB (Universal Serial Bus) interfaces, such as digital still cameras, are becoming common. Such devices can be connected to other electronic devices via USB cables.
Such a portable electronic device is capable of operating by electric power supplied from another electronic device via a USB cable, without using a battery.
According to the USB standard, a current up to 500 mA can be supplied. However, when a portable electronic device is connected to another electronic device via a USB cable, if the power source is simply switched to electric power supplied from the USB cable, operation is not allowed when a current exceeding 500 mA is needed.
Furthermore, in a portable electronic device, when a backup battery (e.g., a secondary battery or a capacitor) is charged during USB suspend, a consumption current supplied from the USB bus becomes greater than or equal to 500 μA, so that the USB standard is not satisfied.
That is, when the current exceeds the value defined by the USB standard, error could occur in signal processing or the like.